At present the so-called Rushton turbomixer, rotated by a shaft centrally arranged in the fermenter and consisting of six rectangular straight blades radially fixed to a circular plate is mainly used in bioreactors (fermenters). If the height of the bioreactor is a multiple of the diameter, a system consisting of 2-6 turbomixers fixed to a common shaft is used.
The air to be dispersed is injected below the lower mixer through a perforated loop expansion pipe, nozzles, or a central nozzle (FEJES, G.: Industrial Mixers, FIG. 46 (p. 52) and FIG. 51 (p. 55).
The turbomixers usually occupy 1/3 of the fermenter diameter and disperse the air efficiently by the intensive turbulence and shear forces generated around the row of blades. However, in consequence to the high local energy dissipation,--despite the high specific power consumption of the turbomixers--the proportion of energy introduced into the zones farther from the mixer is minimal, and the axial transport capacity of the mixer is low, which causes problems especially in the wake of the expanding volume of the bioreactors.
There are also known two-winged or multi-winged propeller mixers with inclined blade or bent according to the geometry of a helical surface, and a mixing system built up from these.
SEM type mixers utilize the flow properties of the thin propeller wings, while EKATO mixers utilize the interference phenomena of the parallel double wing blades arranged at an angle and at the required distance above each other (Interming and Interprop mixers, FEJES, G.: Industrial Mixers FIG. 66, p. 65).
The energy dissipation of propeller mixers with large diameter ratio compared with the diameter of the fermenter (d/D.gtoreq.0.5-0.2) is more uniform, and the axial transport capacity is high. Therefore, with the same power consumption, these devices can mix the liquid more efficiently and evenly in large and narrow fermenters, but their dispersion capacity is weaker. That is intended to be counterbalanced with the use of several phases, and with larger agitator diameter than that of the turbomixers.
Suction mixers consisting of hollow mixing elements fixed to a rotating tubular shaft suitable for mixing, dispersion and partly for transport of the gas are also known. The hollow mixing elements are mostly pipes cut at an angle of 45.degree., at the end of which--at suitable speed--a pressure drop occurs, sucking in the gas usually through the hollow tubular shaft. The gas is atomized by the shear forces generated in the liquid by the sharp pipe-ends (FEJES, G.: Industrial Mixers, p. 57).
These mixers are not used in the fermentation industry because of their limited suction capacity. Suction mixes are also known in which the hollow elements are nearly semi-circulator channels open on the side opposite the direction of displacement, the diameter of which is nearly the same as that of the container, and thus which are suitable for the atomization of relatively large amount of gas (E. Braun: Apparatus for Gasifying Liquids, U.S. Pat. No. 3,092,678). However, because of their low circulation capacity, they are used only in the yeast industry and sometimes in processes not requiring intensive mixing of the liquid.
The purpose of mixing in the reactors is the homogeneous distribution of the solid, liquid and gaseous phases for intensification of the material and heat transfer processes. As a result of mixing, a significant velocity gradient and turbulence are brought about in the space between the mixing elements and the reactor wall provided with buffle plates. In the case of fermentation processes, the velocity-gradient-proportional turbulence and shear forces increase the dispersion of the injected air bubbles, and reduce the thickness of the boundary layers between the microorganisms, culture medium and air bubbles, thereby improving and speeding up the material-transfer and heat transfer processes taking place at the boundary surfaces of the phases.
A three-phase system of the microorganisms, culture medium and injected air is brought about in the bioreactors, where the flow space and its effect on the transfer of material are made extremely complicated by the various interactions, such as change in the rheological properties of the fermenting liquid as a consequence of the metabolism of the microorganisms. The problem is further complicated by diversity and contradiction of the requirements. For example, in a significant number of fermentation processes intensive turbulence and shear are required for dispersion of the air and oil drops, microblending of the culture medium and biomass and cutting up of the agglomerates. At the same time, however, the intensive mixing facilitates the formation of stable foams which partly directly and partly by the foam-inhibiting materials reduces the oxygen transfer and venting of the carbon dioxide, and may mechanically damage the microorganisms, or my bring about production-reducing morphological changes.
It is characteristic to the complexity of the mixing processes taking place in the bioreactors, that each basic operation: dispersion, suspension, dissolution, homogenization, etc. has an important role in the processes. Essentially each fermentation process has its associated specific requirements significantly different according to the type and strain. Thus, the effects of the basic operations should remain within relatively narrow limits in order that--besides the required beneficial effect--the adverse effects should remain minimal. In respect of the turbomixers used in the majority of the bioreactors, it is equally unfavorable to expend the major proportion of the mixing energy for the generation of turbulence. Dissipation of about 70% of the mixing energy takes place in the immediate vicinity of the turbine blades, and these conditions can be changed only in a minor degree.
In the case of fermenting liquids forming intensively aerated viscous and stable foams with non-Newtonian properties, the circulation and turbulence generated by small diameter turbomixers may slow down relatively quickly. The circulation could be intensified with increasing the turbomixer diameter, but this is limited by the disproportionate growth of the mixing power, which--according to the known relationship--increases with the 5th power of the mixer diameter. Therefore, the diameter of the turbomixer must not exceed 40% of the apparatus even in case of small fermenters with capacities below 40 m.sup.3. Thus a characteristic feature of such mixers is the small diameter ratio. On the other hand, this causes additional problems, as the reactor volume and viscosity of the fermenting liquid increase. In this case insufficiently mixed zones appear away from the mixers and the mixed zones become prone to compartmentalization (A. W. Nienow: New Agitators v. Ruston Turbines; Int. Biotech. Symposium (9th, 1992. Crystal City, Va.) pp. 193-196).
The diameter of the propeller mixers--with regard to their much lower rate of power input--may approach the diameter of the reactor. Therefore, use of propeller mixers of high diameter ratio making up 50-70% of the apparatus' diameter is becoming widespread in the bioreactors, the dispersion capacity of which is lower but more suitable for the efficient top-to-bottom mixing of the viscous shear-thinning fermenting broths (B. C. Buckland et al: Biotechnology and Bioengineering, Vol. 31. Pages 737-742 1988)
To provide an efficient mixer is difficult because properties of the viscous fermenting liquids containing microorganisms and air bubbles are often extremely different from those of the Newtonian liquids. Some scientists have found that the turbomixer with smaller diameter is capable of yielding an 8-times higher rate of oxygen absorption, than the turbomixers of greater diameter under the same energy input, although such difference cannot be detected in clear water input, although such difference cannot be detected in clear water (Steel, T. -Maxon, W. D.: Biotechn. and Bioeng. 2, 231, 1962). These not well known phenomena dependent on the properties of cultures and composition of the culture media also justify the build-up of mixing systems, the mixing effect of which can be controlled within wide limits and can be modified in respect of every mixing operation.
On the other hand, a common characteristic of the describe mixers is that they are suitable only for producing a single dominant mixing effect e.g. dispersion, homogenization. Furthermore a common disadvantage is manifested in the fact that the hydrodynamic properties of the mixers can be varied by changing their geometry and rotation speed only to a very limited extent, so that the disadvantages originating from their specific modes of action will remain. These disadvantages could limit optimization of the processes.
The efficiency of mixing high viscosity shear thinning broths in respect of the apparatus depends on the magnitude of the introduced energy and construction of the mixing system. The dissolved oxygen concentration can be improved to the required level generally with the known mixers by increasing the amount of mixing energy and the injected air. However, the disproportionately increasing demand for energy and its cost, intensification of the foam formation and impairment of the microorganisms may limit the economic production as the reactor dimensions increase.
The known multi-stage turbine mixers consist usually of elements of the same shape, and other mixing systems do not provide adequate flexibility for satisfying the specific requirements of the various miroorganisms, due to the mentioned capabilities and restrictions of the construction.
As a consequence of the growing dimensions of the bioreactors, the differences in the functional conditions increase and so too increases the differences in the technical facilities of the mixer stages, which are needed for efficient performance.